1. Field of the Invention
The present invention relates to an image forming apparatus using the electrophotographic process to perform image formation, and more specifically, to an image forming apparatus in which the charges of the photoreceptor surface are removed before the toner image formed on the photoreceptor surface is transferred to a sheet of paper.
2. Description of the Related Art
In image forming apparatuses using the electrophotographic process to perform image formation, generally, after the surface of the photoreceptor is uniformly charged by the charger, the photoreceptor surface is exposed in accordance with data on the original image, thereby forming an electrostatic latent image. Using toner charged to a polarity the same as the polarity of the electrostatic latent image, a potential the same in polarity as the potential of the latent image and higher in level than the potential of the latent image is applied to the developer roller to cause toner to adhere to the photoreceptor where the latent image is formed, so that the latent image is transformed into a visible image (reversal development). Then, the toner image is transferred to a sheet of paper and fixed by heat or pressure. The toner and the latent image charges remaining on the photoreceptor surface are removed by a cleaning unit such as an urethane rubber blade and a charge removing unit performing uniform exposure, respectively.
On the photoreceptor surface having undergone toner development, as shown in FIG. 2(a), a difference .vertline.Vn-Vd.vertline. occurs between the potential Vn of the background area (the part to which no toner adheres) and the potential Vd of the toner surface of the visible image area (the part to which toner (represented by .largecircle. in the figure) adheres). When the difference is large, the formation of the transfer electric field in the transfer process is adversely affected, so that an area in which the electric field is comparatively weak is formed immediately inside the edge of the visible image area. As a result, toner is not easily transferred inside the edge in the central parts of letters and in solidly shaded images, that is, the central parts of letters are missing.
In a conventional image forming apparatus of this type, in order to prevent the central portions of letters from missing or being re-transferred, pre-transfer erasing is performed in which the charges of the photoreceptor surface to which toner adheres are removed before transfer by exposing the photoreceptor surface. For example, in an apparatus described in Japanese Laid-open Patent Application No. H9-101687, the pre-transfer erasing light quantity is set based on the setting of the grid potential of the charger or the potential of the photoreceptor surface detected by the potential sensor, and the charge removal amount is controlled based on the set light quantity. According to this configuration, as shown in FIG. 2(b), the potential of the background area changes from Vn to Vn', so that the difference from the potential Vd of the visible image area decreases. As a result, the central parts of letters are prevented from missing.
If the potential Vn of the background area is decreased to the residual potential level Vr of the photoreceptor surface having passed the charge removing unit before transfer, another problem arises that toner spreads in the periphery of letters at the time of transfer. Therefore, in the conventional example, the pre-transfer potential Vn of the background area is not reduced to as low as the residual potential level Vr. Moreover, in the conventional example, the pre-transfer erasing light quantity is always constant irrespective of the ratio of an image area to a non-image area.
As shown in FIG. 3, in the actual electrophotographic process, image formation processes B1, B2, . . . in which images are formed on the photoreceptor surface and non-image formation processes A, C, . . . , D in which no image formation is performed are repeated like pre-processing A.fwdarw.image formation B1.fwdarw.image interval C.fwdarw.image formation B2.fwdarw.image interval C.fwdarw. . . . .fwdarw.image formation B4.fwdarw.post-processing D. As illustrated in FIG. 5 (showing the photoreceptor surface being developed), when the photoreceptor surface 1a is divided with respect to the rotation direction X, the image formation processes correspond to areas B1 and B2 on the photoreceptor surface 1a with which sheets P1 and P2 are in contact (the areas B1 and B2 will be referred to as "image areas"), whereas the non-image formation processes correspond to areas A, C and D other than the areas B1 and B2 (the areas A, C and D will be referred to as "non-image areas"). Areas G1 and G2 are background areas. Areas M1 and M2 are visible image areas.
Thus, in the actual electrophotographic process, since the image formation processes and the non-image formation processes are repeated, only by setting the pre-transfer erasing light quantity so that the surface potential difference .vertline.Vn-Vd.vertline. between the background areas and the visible image areas in the image areas is reduced without making distinction between the image areas and the non-image areas like in the conventional example, the surface potential is not completely smoothed in the non-image areas A, C and D as shown in FIG. 4(a). That is, in the non-image areas A, C and D, surface potential non-uniformity R1 caused in a stage T1 where the pre-transfer eraser has been passed but the charge removing unit has not been passed yet remains even in a stage T2 where the charge removing unit has been passed (but the charger has not been passed yet) (the residual non-uniformity caused in stage T2 is represented by R2). Consequently, charging non-uniformity R2 is caused in a stage T3 where the charger has been passed for the next image formation (the surface potential non-uniformity caused in stage T3 is represented by R3). Such a potential record (photoreceptor memory) leads to image quality degradation.